1. Field of the Invention
The invention relates to a process for determining dynamic properties of a vehicle, in particular the slip angle of an automobile.
2. Discussion of the Related Art
Automobile racing teams are interested in measuring a variety of vehicle dynamics in order to improve their vehicles' performance. Specifically, racing teams adjust numerous parameters of their vehicles depending on the characteristics of a particular track, e.g., the sloping of a track's curves and the track surface. In fact, many race teams use advanced computer systems to design and make adjustments to their vehicles. See, e.g., Computer Aided Engineering, Vol. 10, No. 5, May 1991, at 20, which provides an overview of the computer-aided design systems used by race teams. The systems typically rely on inputting numerous vehicle and track variables to determine the best combination of vehicle characteristics for increased speed and stability.
One such vehicle characteristic is tires. The tires used on a race car are varied depending on individual track characteristics. In fact, four tires of different properties will typically be used on a single race car. One travel property that significantly affects tire choice is slip angle. As reflected in FIG. 1, which is a top view of a vehicle 10, slip angle is defined as the angle (.alpha.) between the direction the driver is steering (ray X) and the direction the vehicle is traveling (ray Y). Slip angle is a well-known phenomenon, and is discussed, for example, in Milliken et al., Race Car Vehicle Dynamics, SAE International, 1995, the disclosure of which is hereby incorporated by reference. For a particular vehicle and particular tires, individual race tracks will induce different slip angles around the tracks' curves. In fact, since the individual wheels of a race car are typically designed to steer in differing directions during a turn, each wheel may exhibit a different slip angle. Depending on the calculated slip angle, a race team normally adjusts the tire properties, e.g., tread, material, width, diameter, construction, and pressure, to attain a slip angle that provides improved vehicle performance. For this reason, a relatively simple, reliable, real-time measurement of slip angle is desirable.
However, current systems for determining properties such as slip angle are typically complicated, and depend on several distinct sensing devices feeding information to a microprocessor, which then estimates several travel properties, including slip angle. Most of these complex systems were developed by automobile manufacturers in introducing and improving safety systems such as anti-lock brakes and traction control. For such safety systems, the manufacturers are typically interested in sensing and/or calculating a variety of parameters, e.g., yaw (degree of turn about the vertical axis), lateral (side-to-side) acceleration, longitudinal (front-to-back) acceleration, steering angle, and slip angle. These parameters allow adjustments in steering, acceleration, or braking to be quickly and automatically made to control an automobile's motion, e.g., control a skid. See, for example, U.S. Pat. Nos. 4,679,808 ("the '808 patent"), 5,040,115 ("the '115 patent"), and 5,579,245 ("the '245 patent"), all of which use a variety of measured properties to calculate a slip angle value.
The '808 patent discusses a system for determining front and/or rear wheel steering angles necessary to provide desired cornering characteristics. A system described in the patent contains a steering wheel angle sensor, a vehicle speed sensor, a sensor for determining a first motion variable such as yaw rate, a sensor for determining a second motion variable such as yaw acceleration, and a microprocessor. The microprocessor calculates estimated values of the first and second motion variables based on a mathematical vehicle model, on the steering angle and vehicle speed, and on certain vehicle characteristics. In some circumstances, adjustments to the measured first and second motion variables are made based on these estimated values. Then, a third motion variable, such as slip angle, is estimated based on the first and second motion variables and the measured speed and steering angle. The '115 patent similarly measures several properties and inputs the properties to a microprocessor, which then calculates an estimated slip angle based on the inputted data. The '115 patent describes one embodiment containing a longitudinal acceleration monitoring unit, a lateral acceleration monitoring unit, a wheel speed sensor, and an arithmetic circuit for receiving the data. The lateral acceleration data is compared to an experimentally derived slip criterion to calculate the slip angle of a vehicle. The '245 patent utilizes a neural network in an attempt to provide an "actual" slip angle value, as opposed to an estimated value calculated from several measured variables. Specifically, the system of the patent measures front wheel steering, the motion of the vehicle, e.g., velocity, lateral and longitudinal acceleration, and yaw angular velocity, and calculates an estimated slip angle value based on the steering and motion data. The neural network calculates a correction factor in order to provide more accurate estimated slip angle value.
While systems such as those discussed above are useful for estimating a property such as slip angle for purposes of making adjustments to braking, steering, and/or acceleration in a passenger car, such systems are unnecessarily complex when the measurement of primary concern is the slip angle. Moreover, the sensors required in such systems typically will not survive the harsh environment of a racing vehicle. Also, in such previous systems, numerous variables, e.g., lateral acceleration, longitudinal acceleration, and lateral yaw, are relied upon to calculate slip angle, all of which are capable of introducing error into the calculation. Given the number of variables already input into the computer-aided systems used by race teams, a more direct measurement of slip angle would be expected to reduce the overall error introduced into such systems, thereby contributing to improved performance.
Thus, a relatively simple process for reliably and more directly measuring the dynamics of a vehicle, in particular slip angle, is desired.